Brushes such as toothbrushes which are presently on the market are generally formed of a resin such as acrylonitrile-styrene copolymer (hereinafter "AS") or acrylonitrile-butadiene-styrene terpolymer resin (hereinafter "ABS"). The brush filaments which are generally of nylon are set in small holes in the base portion of the brush and are held in these holes or recesses by a thin metal insert which is pressed into the holes.
However, the above mentioned resins which form the base portion of the brush are weak against stress from the outside and strain, and thus tend to become cracked. In addition, these resins are not sufficiently resistant to heat. Consequently, when the toothbrush is disinfected with hot water of 80.degree.-100.degree. C., the filament holding strength of the base portion is decreased with the result that the nylon filaments and the like tend to fall from the holes in which they are inserted.
Attempts have been made to overcome this difficiency by using longer inserts to press more strongly against the sides of the recesses. However, since the brush, and particularly in the case of the toothbrush, is formed of material which is weak against stress and strain, the filament holding recesses or the region between the adjacent filament holding recesses tend to become cracked if the insert is made of a greater length.
Furthermore, if the filament holding recess is made shallower, the brush filament tend to fall more easily from the hole resulting in a low holding strength of the filaments in the base portion of the brush, this being particularly true in the case of the toothbrush.